The present invention relates to a magnetic recording medium, and more particularly, to a magnetic recording medium capable of not only showing a low light transmittance and a low surface resistivity even when the amount of carbon black fine particles added to a magnetic recording layer thereof is as small as possible, but also having a smooth surface by using therein black magnetic acicular composite particles which are excellent in dispersibility in a vehicle due to less amount of carbon black fallen-off from the surface of each black magnetic acicular composite particle, and have a high blackness and a low volume resistivity.
With a development of miniaturized, lightweight video or audio magnetic recording and reproducing apparatuses for long-time recording, magnetic recording media such as a magnetic tape and magnetic disk have been increasingly and strongly desired to have a higher performance, namely, a higher recording density, higher output characteristic, in particular, an improved frequency characteristic and a lower noise level.
Especially, video tapes have recently been desired more and more to have a higher picture quality, and the frequencies of carrier signals recorded in recent video tapes are higher than those recorded in conventional video tapes. In other words, the signals in the short-wave region have come to be used, and as a result, the magnetization depth from the surface of a magnetic tape has come to be remarkably small.
In order to enhance output characteristics of magnetic recording media, especially an S/N ratio thereof with respect to signals having a short wavelength, there have been demanded fineness of magnetic particles, reduction in thickness of a magnetic recording layer, high dispersibility of magnetic particles and surface smoothness of a magnetic coating film.
On the other hand, at the present time, the end position of a magnetic recording medium such as magnetic tapes has been detected by sensing a high light transmittance portion of the magnetic recording medium by means of a video deck. In the case where the particle size of magnetic particles dispersed in the magnetic recording layer become finer and the thickness of the magnetic recording medium is reduced in order to meet the requirement for high performance of the magnetic recording medium as described hereinbefore, the magnetic recording medium shows a high light transmittance as a whole, so that it has been difficult to detect the end position thereof by means of the video deck. In order to solve this problem, carbon black fine particles have been added to the magnetic recording layer in an amount of usually about 6 to 12 parts by weight based on 100 parts by weight of the magnetic particles, thereby reducing the light transmittance of the magnetic recording medium. For this reason, in current videotapes, it is essential to add carbon black fine particles, etc., to the magnetic recording layer thereof.
However, when a large amount of such non-magnetic carbon black fine particles are added to the magnetic recording layer, the magnetic recording medium suffers from deterioration in signal recording property, thereby hindering high-density recording thereon, and the reduction in thickness of the magnetic recording layer becomes incapable. Further, due to the fact that the carbon black fine particles have an average particle size as fine as 0.002 to 0.05 .mu.m and a large BET specific surface area value, and are deteriorated in solvent-wettability, it has been difficult to disperse these carbon black fine particles in vehicle, thereby failing to obtain a magnetic recording medium having a smooth surface.
Therefore, it has been strongly demanded to provide a magnetic recording medium having a sufficiently low light transmittance even when the amount of carbon black fine particles added to the magnetic recording layer is as small as possible, especially when the carbon black fine particles is used in an amount as small as less than 6 parts by weight based on 100 parts by weight of magnetic particles.
Further, in the case where the magnetic recording medium has a high surface resistivity, the electrostatic charge amount thereof is increased, so that cut chips or dusts tend to adhere onto the surface of the magnetic recording medium upon the production or use thereof, thereby causing such a problem that the dropout frequently occurs. Therefore, in order to reduce not only the light transmittance of the magnetic recording medium but also the surface resistivity thereof, especially below about 10.sup.10 .OMEGA./sq, the carbon black fine particles have been conventionally added to the magnetic recording layer of the magnetic recording medium.
However, as described above, in the case where the amount of such carbon black fine particles or the like which do not contribute to magnetic properties of the magnetic recording layer, is increased, there are caused such problems that the magnetic recording medium suffers from deterioration in signal recording property, the reduction in thickness of the magnetic recording layer becomes incapable, and further the surface smoothness of the magnetic recording layer is deteriorated.
Also, since the carbon black fine particles are bulky particles having a bulk density as low as about 0.1 g/cm.sup.3, the handling property and workability thereof are deteriorated. In addition, it has been pointed out that the use of such carbon black fine particles causes problems concerning safety and hygiene such as carcinogenesis.
As the conventional method of reducing the light transmittance and surface resistivity of the magnetic recording medium by lessening the amount of carbon black fine particles added to the magnetic recording layer, there is known a method of increasing the blackness of magnetic particles themselves by incorporating Fe.sup.2+ into magnetic acicular cobalt-coated iron oxide particles in an amount of not less than 6.0% by weight based on the weight of the magnetic acicular cobalt-coated iron oxide particles (Japanese Patent Application Laid-Open (KOKAI) No. 3-102617 (1991) or the like).
However, at present, although it has been most strongly demanded to provide a magnetic recording medium capable of not only showing a low light transmittance and a low surface resistivity even when the amount of carbon black fine particles added to the magnetic recording layer thereof is as small as possible, but also having a smooth surface, such magnetic recording medium which is capable of satisfying all of these properties, cannot be obtained yet.
That is, in the case of the known magnetic recording medium of Japanese Patent Application Laid-Open (KOKAI) No. 3-102617 (1991) in which the magnetic acicular cobalt-coated iron oxide particles containing Fe.sup.2+ in the magnetic particles in an amount of not less than 6.0% by weight are used as magnetic particles, the blackness of the magnetic particles is still unsatisfactory, so that the magnetic recording medium can show neither a sufficiently low light transmittance nor a sufficiently low surface resistivity as described in Comparative Examples hereinafter. Further, since the magnetic particles are deteriorated in dispersibility in vehicle due to the inclusion of Fe.sup.2+, it has been difficult to obtain a magnetic recording layer having a smooth surface.
As a result of the present inventor's earnest studies, it has been found that by using as magnetic particles of a magnetic recording medium, black magnetic acicular composite particles having an average particle diameter of 0.051 to 0.72 .mu.m, which comprise:
magnetic acicular particles as core particles,
a coating layer formed on surface of the magnetic acicular particles, comprising at least one organosilicon compound selected from the group consisting of:
(1) organosilane compounds obtainable from alkoxysilane compounds,
(2) polysiloxanes or modified polysiloxanes, and
(3) fluoroalkyl organosilane compounds obtainable from fluoroalkylsilane compounds, and
a carbon black coat formed on the said coating layer comprising the said organosilicon compound, in an amount of 0.5 to 10 parts by weight based on 100 parts by weight of the said magnetic acicular particles,
the obtained magnetic recording medium can show a low light transmittance and a low surface resistivity, can have a smooth surface due to the fact that the amount of carbon black fine particles added to the magnetic recording layer can be lessened and the dispersibility of the black magnetic acicular particles themselves in vehicle can be enhanced, and, therefore, can be suitably used as a magnetic recording medium for high-density recording. The present invention has been attained on the basis of the finding.